Method for gravity pressure permanent molding

ABSTRACT

The invention is a method to reduce the porosity in gravity pressure permanent molding of metal objects. It involves gravity filling a mold cavity through an opening with a charge of liquid metal that has high solidification shrinkage characteristics. The metal is allowed to slush and then, after a predetermined time, a momentary pressure is applied to the slush by means of pistons operating into the mold while the fill opening is still mechanically unclosed causing the metal to fill voids caused by shrinking. The pistons are then removed preferably before opening the mold. Apparatus for timing the cutoff and piston movement is provided in a structure that is used for making flatiron soleplates.

United States Patent [151 3,685,569 Denton [451 Aug. 22, 1972 [54]METHOD FOR GRAVITY PRESSURE PERMANENT MOLDING Primary Examiner-J.Spencer Overholser Assistant Examiner-John E. Roethel [72] InventorBryce Denmnomanocahf' Attorney-Lawrence R. Kempton, Leonard J. Platt,[73] Asslgnee: General Electric Company John F. Cullen, Frank L.Neuhauser and Joseph B. 22 Filed: July 30, 1910 [2|] Appl. No.: 59,473[57] ABSTRACT The invention is a method to reduce the porosity in 52 us.(:1 ..l64/l20, 164/319 g y pressure permanent molding of metal j 511Int. Cl. .3220 27/10 11 y q gravity filling m cavity through an 581Field of Search ..l64/75, 100, 120, 319, 320, 1 3 2 Y a q s of q me thathas g 164/321 sol1d1ficat1on shrinkage charactensucs. The metal 1sallowed to slush and then, after a predetermined time, [56] ReferencesCited a momentary pressure is applied to the slush by means of pistonsoperating into the mold while the till open- UNITED STATES PATENTS ingis still mechanically unclosed causing the metal to fill voids caused byshrinking. The pistons are then g gi removed preferably before openingthe mold. Ap- 3387646 6,1968 EE; 62/120 paratus for timing the cutoffand piston movement is 1 l t l t d t t th I d f ak' H t 3,344,84810/1967 Hall et al. ..l64/3l2 55:23:2 a S we a use or m mg a 3,270,3839/1966 Hall et al. ..l64/l20 3,106,002 10/1963 Bauer 164/120 2 Clains, 3Drawing Figures LIQUID METAL Patented Aug. 22, 1972 uaum METAL Eyre 4007;?

METHOD FOR GRAVITY PRESSURE PERMANENT MOLDING BACKGROUND OF THEINVENTION 1 Field of the Invention The invention pertains to a methodfor gravity pressure permanent molding of soleplates for electricflatirons to reduce the porosity that normally occurs in the heaviersections of the cast soleplate. This is achieved by using piston-appliedpressure to the cast liquid metal in a particular timed sequence whilethe fill opening remains mechanically unclosed thus resulting in denseand uniform castings that are capable of a high lustre smooth finish.

2. Description of the Prior Art In electric flatirons, it is known toproduce the soleplate by pouring or forcing a molten material such asiron or aluminum alloys into a mold or die cavity in which thetemperature of the enclosure is below the freezing point of the liquid.Such cast objects are frequently found to have holes in the parentmaterial and these are usually caused by entrained bubbles of air or gaspicked up by the molten material during the pouring process.Additionally, gas dissolved in the mo]- ten material is precipitated asbubbles when the parent material freezes and gas is also found toemanate from mold coatings or foreign material on the mold surface orfrom inserts in the casting. Finally, in the case of metal having arelatively high solidification shrinkage characteristic such asaluminum, porosity occurs from the natural shrinkage of the materialbeing cast as it changes from the liquid to the solid state. Such porouscastings are referred to as unsound castings.

A well-known property of aluminum alloys is that upon passing from themolten to the solid state there is about a 7 percent reduction ofvolume. To compensate for this shrinkage, sand molds and permanent moldsare usually provided with risers which are connected to the heaviersections of the casting in such a manner as to be filled with moltenmetal when the mold is filled. The object of the risers is to provide asupply of molten metal which will feed the casting to make up forshrinking as the metal freezes. The risers are seldom fully effectiveand they cause the mold to be complicated in design and are costlybecause they must be cut off or removed and the riser metal must beremelted. In addition, after the riser is cut from the casting theconnecting area is often quite porous. By comparison, in the die castingprocess the outer skin of the part is usually sound because it israpidly chilled by being in contact with the relatively cool die surfacebut the metal midway between the die surfaces is often porous,especially in the thick sections. This occurs in spite of the highpressures used to force the metal into the cavity because the pressureis no longer available to move metal into the cavity as soon as thethinnest part of the gate freezes. When the gate freezes it acts as aplug to prevent the flow of additional matal required to compensate forthe shrinkage caused by freezing of the metal.

It is known to use the concept of plungers wherein the plungers aredesigned to eliminate the shrinkage by packing" the metal at the propertime. Generally this has involved the use of pistons either directlythrough the gate or, if not through the gate, then additional structuremust be supplied to mechanically close the gate so that it is possibleto apply relatively high pressure to the molten or partially solidifiedmetal. This has required the use of rugged and expensive machinerybecause of the pressures involved in handling solidified or partlysolidified metal with the result that the methods and apparatusavailable do not lend themselves to low cost gravity pressure permanentmolding of cast objects which require a high lustre finish such as thatin electric flatiron soleplates.

SUMMARY OF THE INVENTION Briefly described, the invention is directed toa method for making a dense casting of different thickness sections froma metal having relatively high solidification shrinkage which comprisesthe steps of gravity filling a mold cavity through an opening with acharge of liquid metal. The metal is allowed to slush with the fillopening mechanically unclosed except by the slush. While the metal isslushy, a movable piston means, separate from the filling means, appliesmomentary pressure to the slush by moving a predetermined distance intothe mold at the thick sections of the casting while the fill openingremains mechanically unclosed to cause the metal to fill the voidscaused by shrinking. Timed means are provided to actuate the movablemeans assoon as the metal becomes slushy which, in a typical ironsoleplate casting, occurs between 10 to 40 seconds after the filling iscut off. The timed means operates the movable means into the mold forseveral seconds to exert momentary pressure and then withdraw so that nopressure is exerted after the metal solidifies. Thus, the main object isto provide a method that introduces the best features of gravitypermanent molding with the best die casting techniques by using movablepistons in portions of the casting wherein pressure is exerted on thatmetal last to become slushy thereby densifying the metal to producesound castings that are capable of being finished to a high lustre.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional viewshowing the general concept of the gravity permanent molding of theinvention;

FIG. 2 is a schematic view of the apparatus used by the invention; and

FIG. 3 is a perspective view of a typical iron soleplate in its moldshowing the location of the pistons and part of the mold in phantom.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention isapplicable to any cast metal, it is particularly applicable to aluminumalloys which have a high shrinkage solidification characteristic.Consequently, the invention will be described as applied to such alloyas used in electric flatiron soleplates but the invention is not solimited.

In order to visualize the shrinkage that occurs when aluminum alloysfreeze, consider completely filling a mold cavity that is one-half inchin diameter with molten aluminum. If all of the shrinkage were confinedalong the center line when the mold solidified the shrinkage would forma hole more than one-eighth inch in diameter. Another characteristic ofmany aluminum alloys is that they do not suddenly pass from the liquidto the solid state. Many of the alloys, particularly those used inpermanent molding, remain in a slush state for several seconds duringwhich time the metal is sufficiently pliable that a hard object may bepushed into the casting by applying pressure.

Referring to FIG. 1, the invention overcomes the shrinkage problem in amold by providing a movable means such as piston 12 fitted into asuitable hole 14 in the mold and fitted tightly enough to preventleakage of molten metal when the mold is filled through opening 16. At agiven time after the mold is filled the metal becomes slushy and thethinner sections freeze and become relatively hard while the thickersections are still pliable. At this stage, pressure is applied to theouter end of piston 12 causing the piston to move inward displacingsufiicient material to compensate for the reduction in volume of thefreezing metal. This described concept is generally known.

When the timing of the application of pressure by the piston is correctthe metal adjacent to hole 14 is slushy and is shrinking so that whenthe piston starts to move inward a high force is required to pierce theshell of hardened metal which surrounds the slushy core. As soon as theshell is pierced very little additional force is required to move thepiston because the piston is merely filling a space that otherwise wouldbe spongy and largely made up of voids. Thus, with proper timing, thistechnique dispenses with any need for complex mechanical gate closuresand related structure. Consequently, there is no need to close ofi thefill opening at gate 16 because, with this proper timing, the cast metalin the opening has solidified sufficiently to efi'ectively close it andthe piston will not force material up into the gate. Areas of porosityare quite consistent in castings which are produced to identicalconfiguration in high volume such as iron soleplates. Thus, theapplication of pressure at the same thick areas at the proper momentproduces uniformly sound castings.

Referring next to FIG. 2, a schematic apparatus is illustrated whichcomprises a mold in two halves l8 and 20 between which is formed aninternal cavity, as shown in FIG. 3, for producing an aluminum fiatironsoleplate 22. The soleplate is cast around heating element 24 of a knownconstruction and being of the sheathed type. An internal electricalresistance element extends through the sheath with the heating elementseparated from the sheath by an insulating compound resistant to heatsuch as a granulated and compressed magnesium oxide. The heating element24 generally extends in a loop beginning at the rear of the iron asshown and along one side to the forward or nose end and rearwardly alongthe other side. Thus, uniform heat distribution is provided when theiron is plugged in and activated. The soleplate has relatively thick andthin sections so that porosity is found in the thick sections. In orderto produce such a soleplate, a supply container 26 is provided to holdthe liquid metal and this is connected to the mold for gravity fillingby means 28. The metal supply is terminated by a cut-off means 30, whichmay be in the form of a shutter, and which is suitably actuated to blockfilling means 28 at the proper time. In accordance with the invention,it is necessary to provide a movable means equivalent to piston 12 inFIG. 1 to enter the mold at the thick sections and this may take theform of piston means 32 which, in the case of a soleplate, comprises atleast a pair of separate pistons 34 and 36 operable on thick sections ofthe soleplate as shown in FIG. 3. Piston means 32 is connected to asuitable transmission 38 to be moved into the mold at the thick sectionswhereby pistons 34 and 36 are moved a predetermined distance into andthen out of the mold in a timed relation with respect to the cutoff ofthe metal from filling means 28. To this end, an interconnection 40including a clock or triggering mechanism 42 connects the transmissionwith the cut-off means 30.

As previously noted, aluminum alloys of the type used in a soleplate inpermanent molding remain in a slush state for several seconds duringwhich time the metal is pliable.

l have found that by permitting the gate or fill opening 16 to remainopen it will tend to freeze before the center sections or thick sectionsare out of the slushy state. This occurs rather rapidly when the cut-offmeans is operated to end the gravity supply of metal. This is, ineffect, low pressure permanent molding. By tinting the pistons to applypressure to the plastic metal and then removing the pressure after themetal solidifies and doing this after the open or unclosed gatesolidifies, it is possible to compact the thick sections and eliminatethe porosity normally inherent in freezing of the metal. It is importantthat the timing sequence be triggered so that the steps occur in theproper order if satisfactory castings are to be obtained. To this end,the cut-off means 30 is connected to operate trigger means 42 which, inturn, actuates the movable piston means 32 between 10 to 40 secondsafter cut off occurs. The piston means are timed to move into the moldfor several seconds and exert a pressure less than 2,000 psi and thenwithdraw from the mold. Thus, momentary pressure is applied to the slushat the thick sections while the gate opening is mechanically unclosedwhereby the metal fills the porous voids and the pressure is thenremoved. No problems are encountered in closing off the mold because ofthe timed sequence whereby the open pressureless gate permits anyexpansion necessary. Thus, the molten metal is poured into the mold, themetal is allowed to become slush and the open gate to partially freezewhereupon the metal starts to shrink. At this point, makeup metal mayfeed in the gate to take up shrinkage in the interior. When the metalhas partly frozen in the gate then the pistons are moved into the thickslushy sections and momentary pressure is applied. The pistons arepreferably withdrawn to exert no more pressure alter the metalcompletely solidifies and before the mold is opened. It has been foundunnecessary to exert over 2,000 psi for longer then 5 seconds to piercethe shell and then to achieve the dense and solid castings when thistimed sequence is followed. The pressure applying piston movement mayalso be selected so that the final inward location of the piston facesbefore again moving outward will be such that the desired thickness ofthe molded part can be achieved by machining away the excess metal.

In a soleplate it is desirable to use a pair of separate pistons asshown in FIG. 3 directly on selected and different thick sections of thecasting. it has also been found that it is desirable to operate thepistons on the casting side of the soleplate, i.e., the inner side asopposed to the finished bottom surface when the soleplate is polished toreduce any tendency to show by a different grain structure on thefinished soleplate, where the pressure has been applied.

While there has been described a preferred form of the invention,obvious equivalent variations are possible in light of the aboveteachings. It is therefore to be understood that within the scope of theappended claims the invention may be practised, otherwise than asspecifically described, and the claims are intended cover suchequivalent variations.

lclaim:

l. The method of making a dense casting of different thickness sectionsfrom a metal having a relatively high solidification shrinkagecharacteristic comprising the steps of;

gravity filling a mold cavity through an opening with a charge of liquidmetal, allowing said metal to slush with said opening unclosed, applyingmomentary pressure to said slush at the thick sections of said castingwhile said opening is unclosed to cause said metal to fill voids causedby shrinking in said sections, said pressure being applied for up to 5seconds between 10 to 40 seconds after said metal is poured, and thenremoving said pressure before said mold is opened. 2. The methoddescribed in claim 1 wherein the metal is a flatiron aluminum alloysoleplate, and

said pressure is applied at plural locations on the casting side of saidsoleplate.

I I I l

1. The method of making a dense casting of different thickness sectionsfrom a metal having a relatively high solidification shrinkagecharacteristic comprising the steps of; gravity filling a mold cavitythrough an opening with a charge of liquid metal, allowing said metal toslush with said opening unclosed, applying momentary pressure to saidslush at the thick sections of said casting while said opening isunclosed to cause said metal to fill voids caused by shrinking in saidsections, said pressure being applied for up to 5 seconds between 10 to40 seconds after said metal is poured, and then removing said Pressurebefore said mold is opened.
 2. The method described in claim 1 whereinthe metal is a flatiron aluminum alloy soleplate, and said pressure isapplied at plural locations on the casting side of said soleplate.